Extend redcarpet to support a media library 1/2

You're reading the first half of this two-part tutorial that explains how to modify Redcarpet library in order to support images and files of a media library. If you're familiar with RoR or Ruby, you can go straight to part two.

If you use the Markdown syntax for a Rails based blog or website, you probably have a media library with images or files to use in your posts. If Markdown supports external images but there’s no out-of-the box support to tie it to your media using an id or a logical name. Redcarpet - a Ruby library that converts Markdown in HTML - allows overriding the HTML output functions and I’ll present in this tutorial a way to reference your images which will be stored using the Paperclip gem.

Project Initialization

Let’s start by generating a Rails (this tutorial is based on Rails 3.2.1 and bundler 0.9) project, reference needed gems in Gemfile and install them. We’re also using the Paperclip-meta gem to store image width and height.

rails new blog
cd blog
rm public/index.html
echo 'gem "redcarpet"' >> Gemfile
echo 'gem "paperclip"' >> Gemfile
echo 'gem "paperclip-meta"' >> Gemfile
echo 'gem "therubyracer"' >> Gemfile
bundle install

Simple media library ever

Create the Post and Image thanks to Rails’s scaffolding then generate the needed database migrations for Paperclip’s images and their meta-data.

rails g scaffold post title:string body:text published:boolean
rails g scaffold image name:string
rails g paperclip image file
rails g migration AddPaperclipMetaToImage file_meta:text

Add the line has_attached_file to your app/models/image.rb as shown below so each uploaded file will get a thumbnail generated automatically :

class Image < ActiveRecord::Base
  has_attached_file :file, :styles => { :thumb => "212x141>" }

Add Markdown support into the views

Since we’re talking about generating HTML from views, the most appropriate place to plug Redcarpet is a view helper. As the Markdown converter is reused over and over again, we do not want to instantiate it on every request, so we’ll use a class variable to store it. Create the following helper, named app/helpers/redcarpet_helper.rb

module RedcarpetHelper
  def redcarpet_render(content)
    @@markdown ||= Redcarpet::Markdown.new(Redcarpet::Render::HTML, :autolink => true)

Adjust the app/view/posts/show.html.erb to use the Redcarpet renderer from the helper we created :

<p id="notice"><%= notice %></p>

  <%= @post.title %>
  <%= redcarpet_render @post.body %>
  <%= @post.published %>

<%= link_to 'Edit', edit_post_path(@post) %> |
<%= link_to 'Back', posts_path %>

Modify app/views/images/_form.html.erb to allow images upload :

<%= form_for(@image) do |f| %>
  <% if @image.errors.any? %>
    <div id="error_explanation">
      <h2><%= pluralize(@image.errors.count, "error") %> prohibited this image from being saved:</h2>

      <% @image.errors.full_messages.each do |msg| %>
        <li><%= msg %></li>
      <% end %>
  <% end %>

  <div class="field">
    <%= f.label :name %><br />
    <%= f.text_field :name %>
  <div class="field">
    <%= f.label :file %><br />
    <%= f.file_field :file %>
  <div class="actions">
    <%= f.submit %>
<% end %>

Edit app/views/images/show.html.erb to add the image_tag to display uploaded images :

<p id="notice"><%= notice %></p>

  <%= @image.name %>

<%= image_tag @image.file.url(:original) %>

<%= link_to 'Edit', edit_image_path(@image) %> |
<%= link_to 'Back', images_path %>

Now let’s run the database migration and run the rails server

bundle exec rake db:migrate
rails s

You can now connect to http://localhost:3000/images and http://localhost:3000/posts to upload images and create content.

In the next part of this tutorial, we’ll see how to use uploaded images withing Markdown content.

See also